The present invention relates to a multi-layer metal tube, especially for brake lines of motor vehicles, that is made by winding or cylindrically rolling a flat steel band and subsequently soldering (brazing) with a soldering (brazing) layer provided at least on one side of the steel band.
The invention further relates to a method for manufacturing a multi-layer metal tube, especially for brake lines of motor vehicles, in which a steel band which is provided at least on one side with a soldering layer is wound or cylindrically rolled and subsequently soldered to form a pressure-tight tube by heating to a temperature above the melting temperature of the soldering layer.
Multi-layer metal tubes that are manufactured by winding or cylindrically rolling a flat steel band are especially employed in the automobile industry, especially for brake lines for motor vehicles. A further application of such tubes is their use in refrigeration machines.
From German publication 854 608 a multi-layer tube, manufactured from a flat steel band that is wound helically and subsequently soldered, is known in which as the binding metal between the steel layers that are laying atop one another a copper layer is used which has been applied prior to winding or rolling in a galvanic process. With this technique for manufacturing tubes, which has been used for a long time, problems have arisen from the beginning because such tubes have only an insufficient corrosion resistance. Therefore, these tubes, especially with respect to their use in brake lines of motor vehicles, have decisive disadvantages. This is so because brake lines are subjected to strong corrosive media, such as street dirt, moisture, and salt solutions. Furthermore, brake lines are subject to strong vibrations.
Multi-layer tubes have been substantially improved in the past by providing them with a zinc layer. Also, in the past years tubes have been additionally olive-chromized and coated with liquid plastic material.
As a further measure for improving such tubes it has been suggested to use steel bands made of stainless steel and especially austenitic chromium-nickel steels, as is, for example, described in German patent 40 10 178. The steel alloy suggested in this patent has a contents of 2 to 2.5% molybdenum and a contents of at most 2% manganese. The soldering material is a copper layer which is used in a special soldering method in which certain time and temperature limits must be precisely observed. In a first step, the tube is heated to a temperature of 1000.degree. C., and in a further method step is slowly heated to the melting temperature of the copper soldering material. In a third method step, the metallurgical connection between the copper soldering material and the steel band takes place, whereby in this method step a time period of 20 seconds must not be exceeded. The observance of these precisely defined method parameters is necessary in order to prevent the risk of grain boundary diffusion of the soldering material which is an undesirable phenomenon that can occur during soldering. When such a diffusion occurs, soldering discontinuities may result leading to leaks in the tubes. The observance of the short heating period, however, is rather difficult because of the good heat conductivity of the tubes and the high manufacturing speeds of 100 to 200 m per minute. The grain boundary diffusion of copper further leads to a considerable reduction of the corrosion resistance of the tubes.
It is therefore an object of the present invention to provide a multi-layer metal tube, and especially a multi-layer metal tube for brake lines of motor vehicles, with which leaks and corrosion can be substantially reduced. Furthermore, a manufacturing method for such a multi-layer metal tube will be suggested.